1. Field of the Invention
This invention relates to cooling structures that cool down integrated circuit devices mounted on multichip modules by using heat sinks as well as methods of manufacturing those structures. This application is based on patent application No. Hei 9-174143 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
According to the conventional cooling structure of the multichip module, heat-radiation surfaces of devices of LSI (an abbreviation for "Large Scale Integration") mounted on a board of the multichip module are forced to come in contact with a single heat sink. Thus, it is possible to radiate heat from each of the LSI devices. In some cases, however, the LSI devices differ from each other in standards. Or, even if all of the LSI devices are manufactured in the same standard, the LSI devices differ from each other due to manufacture error, which occurs when they are mounted on the board, or due to a bend of the board and the like. In that case, uniform mount height cannot be secured for all of the LSI devices, whose mount heights may disperse. For this reason, the LSI devices may have different slopes in upper surfaces thereof. The dispersion in mount heights of the LSI devices may frequently reach 0.5 mm or so. The dispersion in mount heights and slopes of the LSI devices may cause dispersion in heat-radiation surfaces and slopes of the LSI devices. So, there exist LSI devices whose heat-radiation surfaces cannot come in contact with the heat sink. As a result, there occurs a problem that those LSI devices cannot radiate heat.
To solve the above problem, a heat-transfer sheet having flexibility is provided in a gap between the heat-radiation surface of the LSI device and a lower surface of the heat sink so as to absorb (or cancel) dispersion in heights and slopes of the heat-radiation surfaces. Such a technique will be called "prior art 1".
The paper of Japanese Patent Publication No. Sho 62-59887 discloses another technique (called "prior art 2") that provides bond having good thermal conduction in a gap between the heat-radiation surface of the LSI device and the lower surface of the heat sink so as to absorb dispersion in heights and slopes of the heat-radiation surfaces.
However, the prior arts 1 and 2 have a problem because they cannot be applied to the case where the LSI devices mounted on the multichip module have great heating values.
The aforementioned heat-transfer sheet and thermal conductive bond, used by the prior arts 1 and 2, contain thermal conductive particles such as alumina-filler. In order to achieve heat radiation of the LSI device having a great heating value, a number of the thermal conductive particles is increased to increase thermal conductivity. In that case, however, flexibility could be damaged in the heat-transfer sheet and the thermal conductive bond before hardening, because, the density of the thermal conductive particles is increased. If the flexibility is damaged, it should be necessary to impart excessive stress to the heat-radiation surfaces to absorb dispersion in mount heights of the heat-radiation surfaces. That is, sufficient flexibility should be required for the heat-transfer sheet and the thermal conductive bond before hardening. For this reason, it is difficult to increase the number of the thermal conductive particles. So, there is a limit in thermal conductivity, which corresponds to 1 W/m.multidot.K or so. As described above, the heat-transfer sheet and thermal conductive bond of the prior arts 1 and 2 cannot reduce thermal resistance.